1. Field of the Invention
The present invention relates to optical fibers made from halide glasses which are transparent in the visible to infrared region and applicable to optical communication, optical measurement and laser light transmission.
The present invention also relates to optical fibers having a core glass doped with rare earth ions as activating species and applicable to optical media for amplifying the light in the visible to infrared region, optical materials for constituting a laser unit and the like.
The present invention further relates to optical fibers for the light amplification which are applicable to optical communication systems and optical amplifiers incorporating such optical fibers. According to the present invention, optical fibers for the light amplification and optical amplifiers incorporating such optical fibers advantageously exhibit an elevated gain coefficient.
2. Related Art
Transparency limit of glasses in long wavelength region is decided according to phonon energy of the glasses. That is, a glass with a smaller phonon energy have a transparency limit at a longer wavelength. Fluoride glasses based on zirconium fluoride (ZrF.sub.4) are known as glass materials having good transparency properties in the visible to infrared region. These fluoride glasses exhibit an excellent devitrification resistance. However, Zr ions strongly attract anionic ions due to their relatively small atomic weight and high valence (tetravalent). As a result, these zirconium fluoride glasses have about 550 cm.sup.-1 of phonon energy (h.omega./2.pi.) and absorb the light of about 4-5 .mu.m or longer. Thus use of these glasses is restricted in the longer wavelength region.
It has been known that, when crystals or glasses doped with rare earth ions are used as laser materials, non-radiative relaxation rate (Wnr) due to the multiphonon relaxation is generally represented by the following equation: EQU Wnr=Wnr(0).multidot.exp-.alpha..multidot..DELTA.E/(h.omega./2.pi.)!
In the above equation, Wnr(0) and .alpha. are constants inherent in materials, .DELTA. E is an energy gap between the emission level (excitation level) and an another level immediately below the emission level and h.omega./2.pi. is an maximum phonon energy of glasses. Among these factors, it is h.omega./2.pi. that predominantly influences on the Wnr of glasses. In other words, it can be said that, in glasses having a larger h.omega./2.pi., non-radiative relaxation rate becomes larger and radiative quantum efficiency is lowered.
The above-mentioned zirconium fluoride glasses based on ZrF.sub.4 have the drawback that, since they have the value of h.omega./2.pi. as high as about 550 cm.sup.-1, the rare earth ions doped therein have a lower radiative quantum efficiency as calculated by the above equation (see R. S. Doel et al., Journal of Non-Crystalline Solids, 161, 1993, p.257).
An optical fiber is generally composed of a core and a cladding, the latter having a lower refractive index. When the refractive index of core is designated as n.sub.1, and that of cladding is designated as n.sub.2,.DELTA.n in the following equation is called the specific refractive index difference: EQU .DELTA.n=(n.sub.1 -n.sub.2)/n.sub.1 !.times.100(%)
The numerical aperture (N.A.) is defined as follows: EQU N.A.=(n.sub.1).sup.2 -(n.sub.2).sup.2 !.sup.1/2
From the relations set forth above, it might be said that a fiber having a larger specific refractive index difference has also a larger numerical aperture.
The numerical aperture is the measure of the angle within which incident light can be caught by the fiber. In other words, it is the measure of the maximum incident angle of light which can propagate within the fiber. A fiber having a larger numerical aperture may permit much light to be entered into the fiber. Thus, the fibers with an elevated numerical aperture and an increased specific refractive index difference are preferred when applied in optical measurement, laser light transmission and the like,
Fibers having a core doped with rare earth ions are expected to be a promising material for fabricating fiber lasers or fiber amplifiers. That is to say, by imparting to such fibers an increased numerical aperture and an increased specific refractive index difference, it will become possible to confine the propagated light energy to the smaller core. Accordingly, an increased numerical aperture and an increased specific refractive index difference will be preferred for obtaining an improved efficiency of lasing and amplification.
As known methods for increasing the specific refractive index difference and the numerical aperture of a fiber made from halide glass, fluoride glass and the like, there are (1) a method for decreasing the refractive index by adding Hf to a cladding, and, (2) a method for increasing the refractive index by adding heavy metals such as Pb ions to a core (Japanese Patent Laid Open (JP-A-) Nos. 6-24791 and 6-69584).
The fiber disclosed in the above 6-24791 has a specific refractive index difference (.DELTA.n) of 3.5% at maximum.
The 6-69584 reference discloses the fiber which may have theoretically the maximum value of 5.4% as the specific refractive index difference (.DELTA.n). However, this value was obtained by preparing a core glass and a cladding glass separately and calculating the specific refractive index difference (.DELTA.n) of the fiber from the respective refractive indices of these glasses. In fact, the maximum value of the specific refractive index difference (.DELTA.n) which was measured practically on the obtained fiber was only 3.7%.
The fibers described in the above documents contain a core made from glass based on zirconium fluoride. The optical fiber amplifiers operating at 1.3 .mu.m range generally require the optical fibers doped with Pr ions as activating species. The problem is that the radiative quantum efficiency at 1.3 .mu.m range obtained by the fluoride glass based on zirconium fluoride doped with Pr ions is lower than that obtained by other fluoride glasses based on indium fluoride optionally with gallium fluoride and doped with Pr ions (see S. Doel, ibid.).
By increasing the specific refractive index difference (.DELTA.n) by 1%, for example by increasing from 3% to 4%, one could expect an increase of about 20% to 30% in efficiency (Japanese Patent Laid Open No.6-69584). A core-forming glass having a high refractive index and a clad-forming glass having a low refractive index could be fabricated independently. However, fiber formation from these glasses was not always easy due to the differences of characteristics between these glasses, such as glass transition temperatures, crystallization temperatures and so on.
Therefore, an object of the present invention is to provide an optical fiber having a large numerical aperture and a specific refractive index difference (.DELTA.n) of at least 4%, preferably at least 5.5%, which comprises a core and a cladding, both being constituted by fiber-formable glasses, and the core being composed of a glass having a high quantum efficiency of activating ions such as Pr.sup.3+.
Another object of the present invention is to provide an optical fiber described above wherein the core contains an activating species, and is applicable to fiber lasers, optical amplifiers and others.
Further object of the present invention is to provide an optical amplifier made from an optical fiber with a core containing an activating species.